Tellurium Dioxide, TeO2

As has already been mentioned, tellurium burns in air with the formation of the Tellurium Dioxide, TeO2.

The oxidation can also be effected in the wet way, for example by the gradual addition of finely divided tellurium (preferably precipitated) to excess of nitric acid. A basic tellurium nitrate is the primary product, but on suitably diluting with water, tellurium dioxide is obtained as a colourless, crystalline precipitate. An alternative procedure is to evaporate the nitric acid solution and ignite the residue.

Tellurium dioxide is known in two different crystalline forms. Crystals of the tetragonal system, but almost regular (a:c = 1:1.1076), of density 5.66, are obtainable from the solution in nitric acid, while the molten dioxide when slowly cooled deposits rhombic needles (a:b:c = 0.4566:1:0.4693) of density 5.93 and identical with the rarely occurring mineral tellurite.

The formation of tellurium dioxide from tellurium and oxygen is attended, according to Mixter, by the evolution of 87,100 calories per gram-molecular weight. Schuhmann gives the heat of formation at 25° C. as 77,700 calories and the free energy of formation as -64,320 calories.

When heated, the dioxide fuses at an incipient red heat, giving a clear, deep yellow liquid, the colour being lost on cooling. On account of its considerable latent heat of fusion, the mass becomes feebly incandescent during solidification. Appreciable volatilisation occurs at 400° to 500° C., the oxide, however, being much less volatile than tellurium itself.

Tellurium dioxide is very sparingly soluble in water (1 part in 150,000 at the ordinary temperature). The solution has no acidic properties. The presence of even small quantities of nitric acid raises the solubility, on account of the formation of a basic nitrate, hot dilute solutions of which, however, frequently deposit the dioxide in a crystalline form on cooling. It is also soluble in diluted sulphuric acid, owing to the formation of basic telluric sulphate.

The dioxide can be reduced to tellurium by heating with carbon or with potassium cyanide. A similar result can be produced by heating in a current of hydrogen, but the temperature required is high.

When heated in sulphur monochloride vapour, tellurium dioxide is readily attacked with formation of tellurium tetrachloride or dichloride, according as to whether the sulphur monochloride or the tellurium dioxide is present in excess.

Aqueous solutions of the alkali hydroxides readily dissolve tellurium dioxide with formation of the corresponding tellurite. In the presence of hydrogen peroxide the corresponding tellurate is formed. Ammonia and the alkali carbonates in cold aqueous solution have little effect, but the latter in hot solution or in the fused condition give rise to tellurites. Nitrates of the alkali metals on fusion with tellurium dioxide produce tellurates.

Tellurium Dioxide as a Base

Tellurium dioxide possesses definite basic tendencies, the tellurium being capable of acting as a quadrivalent atom and the group TeO, telluryl, as a bivalent basic radical.

When heated in a current of dry hydrogen chloride or with solid ammonium chloride, tellurium dioxide forms the tetrachloride. With hydrogen chloride at low temperatures several workers have reported the formation of additive compounds similar to those obtained with selenium dioxide; these at higher temperatures yield water and the tetrachloride. Parker and Robinson, however, in a recent investigation find no evidence that tellurium dioxide forms any definite addition compound up to 150° C.; at 0° C. hydrogen chloride is absorbed to give a product obviously not homogeneous, which loses water continuously with rise in temperature, and probably consists of basic chlorides. The formation of the tetrabromide by the interaction of tellurium dioxide and hydrogen bromide has already been described.

The salts of basic tellurium are colourless and are easily hydrolysed in dilute aqueous solution with formation of the dioxide. The addition of tartaric acid checks the separation of the dioxide on account of the formation of the stable acid tellurium tartrate Te(HC4H4O6)4,which can be obtained in the crystalline condition, as also can the silver telluryl tartrate Ag2(TeO)(C4H4O6)2, analogous to potassium antimonyl tartrate.

By treatment with sufficiently strong oxidising agents, such as chromic acid or potassium permanganate with hydrochloric or sulphuric acid, aqueous solutions of salts of basic tellurium are converted into solutions of telluric acid.

Tellurium Dioxide as an Acid Anhydride

When tellurium tetrachloride is treated with water, or when an aqueous solution of a tellurium salt is decomposed by an aqueous alkaline solution, a bulky, colourless precipitate is obtained which is sufficiently soluble in water to impart an acid reaction to the solution and which is more readily soluble than tellurium dioxide in acids or alkalis. A similar precipitate is obtained on acidifying a cold aqueous solution of potassium tellurite with a slight excess of nitric acid. This product is a tellurous acid, possibly H2TeO3, but it is very unstable and spontaneously dehydrates, slowly at the ordinary temperature and rapidly at 40° C. with formation of dioxide.

Tellurous acid can be prepared from the residues from the electrolytic refining of copper by treating them with a solution of ammonia. On the addition of acetic acid to the resulting solution tellurous acid is obtained as a precipitate. When this precipitation is carried out in the cold the product obtained is readily soluble in alkali hydroxide, but if the precipitation takes place in a hot solution the product tends to be insoluble in the alkali hydroxides.

The concentration of tellurous cation in solutions containing increasing amounts of hydrochloric acid has been determined by potential measurements, using a tellurium electrode, the total tellurium content of the solutions being determined chemically. The concentration of the tellurium ion was found to increase as the fourth power of the concentration of the hydrogen ions, the relation being in accordance with the expression

= 1.5×10-46

This is in agreement with the behaviour of tellurous acid as a weak base.